Exhaust gas treatment system with emission control during filter regeneration

ABSTRACT

An exhaust gas treatment system for an engine includes an exhaust gas inlet tube configured to receive an exhaust gas from the engine. A particulate filter, a heat exchange system and first and second selective catalytic reduction (SCR) devices are in fluid communication with the exhaust gas inlet tube. The particulate filter is configured to undergo thermal regeneration when the exhaust gas in the particulate filter is heated above a regeneration temperature. The controller is configured to control a temperature difference, between a present temperature of the second SCR device and a predefined optimal second SCR temperature, to be within a predefined threshold during the thermal regeneration of the particulate filter. The controller may be configured to direct an injector to inject a reductant into the first SCR device when the temperature difference is below the predefined threshold, thereby controlling a NOx emission in the exhaust gas.

TECHNICAL FIELD

The present invention relates generally to an exhaust gas treatment system for a vehicle and a method for controlling the exhaust gas treatment system.

BACKGROUND

Internal combustion engines produce a number of emissions, including various oxides of nitrogen, referred to collectively herein as NOx gases. NOx gases are created when nitrogen and oxygen molecules present in engine intake air are exposed to high temperatures of combustion. Exhaust gas treatment systems are used in vehicles in order to reduce and manage the NOx gases created in the combustion process. Exhaust gas treatment systems generally employ a selective catalytic reduction (SCR) device which uses a reductant, such as ammonia, capable of reacting with NOx gases in combination with excess oxygen in order to reduce the NOx gases.

Exhaust gas treatment systems also employ particulate filters to filter out particles or particulate matter produced by the engine. On regular intervals, the particulate filter has to be thermally regenerated in order to remove the accumulated particles. As the temperature of the particulate filter is increased, the temperature of the SCR device is also increased, resulting in ammonia being desorbed from the SCR device. The ammonia may pass through the particulate filter and be oxidized to form NOx gases, thereby increasing NOx emissions during the thermal regeneration of the particulate filter.

SUMMARY

An exhaust gas treatment system for an engine producing an exhaust gas includes an exhaust gas inlet tube configured to receive the exhaust gas from the engine. A particulate filter, a heat exchange system and first and second selective catalytic reduction (SCR) devices are in fluid communication with the exhaust gas inlet tube. The heat exchange system is positioned downstream of the particulate filter. The first and second SCR devices are positioned upstream and downstream of the heat exchange system, respectively. The particulate filter is configured to undergo thermal regeneration when the exhaust gas in the particulate filter is heated above a regeneration temperature. A first temperature sensor is operatively connected to the second SCR device and configured to determine a present second SCR temperature (T_(S2)) of the second SCR device. A controller is operatively connected to the first temperature sensor and configured to determine whether the thermal regeneration is taking place in the particulate filter. The controller is configured to control the temperature difference (T_(S2)−T_(O)) between the present second SCR temperature (T_(S2)) and a predefined optimal second SCR temperature (T_(O)) to be within a predefined threshold during the thermal regeneration of the particulate filter.

An injector is operatively connected to the first SCR device and configured to selectively inject a reductant into the first SCR device. The reductant is configured to travel to the second SCR device. The controller may be configured to direct the injector to inject the reductant when the temperature difference (T_(S2)−T_(O)) is below a predefined threshold, thereby controlling the NOx emission in the exhaust gas during the thermal regeneration of the particulate filter. In one example, the predefined optimal second SCR temperature (T_(O)) is between approximately 200 and 220° Celsius. In another example, the predefined optimal second SCR temperature (T_(O)) is approximately 220° Celsius and the predefined threshold is approximately 10° Celsius.

The controller being configured to control the temperature difference (T_(S2)−T_(O)) to be within a predefined threshold includes directing the heat exchange system to transfer heat from the exhaust gas when the temperature difference (T_(S2)−T_(O)) is above the predefined threshold. Thus, the exhaust gas treatment system uses the heat exchange system to control the present second SCR temperature (T_(S2)) of the second SCR device during the thermal regeneration of the particulate filter.

First and second pressure sensors may be positioned upstream and downstream of the particulate filter, respectively. The first and second pressure sensors are configured to determine a differential pressure across the particulate filter. The controller may be configured to determine whether the thermal regeneration is taking place in the particulate filter by determining when the differential pressure across the particulate filter is above a predefined threshold pressure.

A second temperature sensor is operatively connected to and configured to determine a present filter temperature (T_(F)) of the particulate filter. The controller may be configured to determine whether the thermal regeneration is taking place in the particulate filter by determining whether the present filter temperature (T_(F)) of the particulate filter has elapsed a predefined amount of time at a predefined temperature. In one example, the predefined amount of time is 30 minutes and the predefined temperature is 550 Celsius.

The first SCR device may include a first catalyst and the particulate filter may include a plurality of channels having respective walls. The first SCR device and the particulate filter may be disposed in a common housing such that the first catalyst is coated on the respective walls of the plurality of channels of the particulate filter. First and second NOx sensors may be positioned upstream and downstream of the particulate filter, respectively. The first and the second NOx sensors are configured to determine respective amounts of NOx in the exhaust gas upstream and downstream of the particulate filter.

The heat exchange system may include an inlet portion configured to receive the exhaust gas from the particulate filter. An outlet portion of the heat exchange system is configured to transmit the exhaust gas to the second SCR device. An interior cavity connects the inlet and outlet portions and defines a central passageway and a bypass passageway. A heat exchange device is positioned within the bypass passageway and configured to transfer heat from the exhaust gas.

A bypass valve is selectively movable between a plurality of positions to selectively permit the exhaust gas entering the second SCR device to include a first portion from the central passageway and a second portion from the bypass passageway. The bypass valve may be positioned such that the first portion is approximately 100% and the second portion is approximately 0% when the temperature difference (T_(S2)−T_(O)) is below the predefined threshold. The bypass valve may be positioned such that the first portion is approximately 60% and the second portion is approximately 40% when the temperature difference (T_(S2)−T_(O)) is above the predefined threshold.

A coolant circuit may be operatively connected to the heat exchange system such that the heat exchange device is configured to selectively transfer heat from the exhaust gas to the coolant circuit. A method for controlling operation of the exhaust gas treatment system is provided.

The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary exhaust gas treatment system and a controller which uses an algorithm as set forth herein;

FIG. 2 is a schematic flow diagram for an algorithm or method for controlling the exhaust gas treatment system shown in FIG. 1; and

FIG. 3 is a schematic perspective view of an example heat exchange device that may be employed in the exhaust gas treatment system of FIG. 1.

DETAILED DESCRIPTION

Referring to the drawings, wherein like reference numbers correspond to like or similar components throughout the several figures, a portion of a vehicle 10 is shown in FIG. 1 having an engine 12 producing an exhaust gas 14. In one example, the engine 12 is a diesel engine. However, the disclosure is applicable to any type of engine. The vehicle 10 includes an exhaust gas treatment system 16 for treating constituents in the exhaust gas 14 such as oxides of nitrogen (NOx). An exhaust gas inlet tube 18 is in fluid communication with and configured to receive the exhaust gas 14 from the engine 12.

Referring to FIG. 1, the treatment system 16 includes a particulate filter 20 in fluid communication with the exhaust gas inlet tube 18. A heat exchange system 22 is in fluid communication with the exhaust gas inlet tube 18 and positioned downstream of the particulate filter 20. A first selective catalytic reduction (SCR) device 24 is in fluid communication with the exhaust gas inlet tube 18 and positioned upstream of the heat exchange system 22. A second selective catalytic reduction (SCR) device 26 is in fluid communication with the exhaust gas inlet tube 18 and positioned downstream of the heat exchange system 22. The first and second SCR devices 24, 26 are aimed at reducing oxides of nitrogen (NOx) in the exhaust gas 14 by conversion to nitrogen and water vapor. The first and second SCR devices 24, 26 use a reductant 28 capable of reacting with NOx in combination with excess oxygen. The reductant 28 may be urea, ammonia, an ammonia precursors or any other suitable material. In one example, the reductant 28 is diesel exhaust fluid (DEF).

Referring to FIG. 1, an injector 29 is operatively connected to the first SCR device 24 and configured to selectively inject the reductant 28 into the first SCR device 24. The reductant 28 is configured to travel to the second SCR device 26, through the particulate filter 20 and the heat exchange system 22. Alternatively, a second injector (not shown) may be operatively connected to and configured to inject the reductant 28 into the second SCR device 26.

Referring to FIG. 1, a mixer 30 may be fluidly connected to the exhaust gas inlet tube 18. The mixer 30 may be positioned in close proximity to the injector 29 to enable through mixing of the reductant 28 with the exhaust gas 14. The mixer 30 may include longitudinally-oriented channels that allow the exhaust gas 14 and the reductant 28 to mix prior to entering the first SCR device 24.

The NOx reduction reaction takes place as the exhaust gas 14 passes through the first and second SCR device 24, 26. Referring to FIG. 1, the second SCR device 26 includes a carrier or substrate 34 that is dipped into a washcoat containing an active catalytic component, referred to herein as second catalyst 36. The second catalyst 36 is coated onto the substrate 34. The second catalyst 36 may be an oxide of a base metal such as vanadium, molybdenum, tungsten and zeolite. In one example, the second catalyst 36 is an iron- or copper-exchanged zeolite. For maximum efficiency, the second catalyst 36 requires an optimal temperature within the second SCR device 26. The substrate 34 is configured to increase the surface area available for coating of the second catalyst 36. The substrate 34 may be composed of a ceramic honey-comb like block, metal or any other suitable material. The substrate 34 may be supported with a metallic or mineral ‘mat’ (not shown) and then packaged into a container 38. The container 38 may be a stainless steel can. Any substrate 34 may be employed in the second SCR device 26.

The particulate filter 20 is used to filter out particles or particulate matter produced by the engine 12. These particles may include soot, hydrocarbons, ashes and sulphuric acid. Referring to FIG. 1, the particulate filter 20 may include a plurality of channels 40 which are one-ended and have respective porous walls. The exhaust gas 14 travels through the porous walls of the channels 40, as shown by arrow 42, leaving particles filtered on the walls of the channels 40. The channels 40 may be composed of ceramic or any other suitable materials.

Referring to FIG. 1, the first SCR device 24 includes an active catalytic component, referred to herein as first catalyst 44. The first catalyst 44 may be an oxide of a base metal such as vanadium, molybdenum, tungsten and zeolite. In one example, the first catalyst 44 is a copper-exchanged zeolite. The first SCR device 24 and the particulate filter 20 may be disposed in a common housing 46 such that the first catalyst 44 is coated on the respective walls of the channels 40 of the particulate filter 20.

The exhaust treatment system 16 includes one or more sensors at various locations for sensing the temperature, pressure and other properties of the system 16. Referring to FIG. 1, a first temperature sensor 48 is operatively connected to the second SCR device 26 and configured to determine a present temperature (referred to herein as “present second SCR temperature T_(S2)”) of the second SCR device 26. A second temperature sensor 50 is operatively connected to and configured to determine a present filter temperature (T_(F)) of the particulate filter 20. First and second NOx sensors 52, 53 may be positioned upstream and downstream of the particulate filter 20, respectively. The first and the second NOx sensors 52, 53 are configured to determine respective amounts of NOx in the exhaust gas 14 upstream and downstream of the particulate filter 20. First and second pressure sensors 54, 56 may be positioned upstream and downstream of the particulate filter 20, respectively. The first and second pressure sensors 54, 56 are configured to determine a differential pressure across the particulate filter 20.

Referring to FIG. 1, the exhaust gas treatment system 16 may include an oxidation catalyst 58. The oxidation catalyst 58 is located upstream of the particulate filter 20. Exhaust gas 14 from the engine 12 passes through the oxidation catalyst 58, and into the first SCR device 24. The oxidation catalyst 58 converts the NO (nitrogen monoxide) gas into NO₂, which is easily treated in the first SCR device 24. The oxidation catalyst 58 also eliminates some sulphur derivatives and other compounds from the exhaust gas 14 by oxidizing them to other compounds. As the oxidation catalyst 58 oxidizes the hydrocarbon emissions in the exhaust gas 14, heat is released due to the exothermic nature of the reactions. This heat may be used to complete the regeneration of the particulate filter 20, as described below.

On regular intervals, the particulate filter 20 has to be regenerated in order to remove the accumulated particles. The particulate filter 20 is configured to undergo thermal regeneration when the exhaust gas 14 in the particulate filter 20 is heated above a regeneration or combustion temperature, thereby allowing the particles to combust or burn. In one example, the regeneration temperature is between 600-750° C. Any suitable method of performing regeneration may be employed, including but not limited to, using a fuel burner, using resistive heating coils and using microwave energy. As the temperature of the particulate filter 20 is increased, the temperature of the first SCR device 24 is also increased, resulting in the reductant 28, such as ammonia, being desorbed from the first SCR device 24. The ammonia may pass through the particulate filter 20 and be oxidized to form NOx gases (various oxides of nitrogen), thereby increasing NOx emissions during thermal regeneration of the particulate filter 20.

Referring to FIG. 1, a controller 60 is operatively connected to the engine 12 and other components of the vehicle 10. Controller 60 is configured to minimize NOx emissions in the exhaust gas 14 during the thermal regeneration of the particulate filter 20. Controller 60 does so by executing an algorithm 200 which resides within the controller 60 or is otherwise readily executable by the controller 60. The controller 60 may be microprocessor based such as a computer having a central processing unit, memory (RAM and/or ROM), and associated input and output buses. The controller 60 may be an application-specific integrated circuit or may be formed of other logic devices known in the art. The controller 60 may be a portion of a central vehicle main control unit such as the engine control module (ECM), an interactive vehicle dynamics module, a main control module, a control circuit having a power supply, combined into a single integrated control module, or may be a stand-alone control module.

Execution of algorithm 200 is described below with reference to FIG. 2. The start and exit functions are denoted in FIG. 2 as “S” and “E”, respectively. It is to be appreciated that the controller 60 may eliminate one or more steps or may determine the steps in an order other than as described above. Algorithm 200 may begin with step 202, where the controller 60 of FIG. 1 determines whether the thermal regeneration is taking place in the particulate filter 20. This may be done in multiple ways. In one embodiment, the controller 60 may be configured to determine whether the thermal regeneration is taking place in the particulate filter 20 by determining when a differential pressure across the particulate filter 20 (as determined by first and second pressure sensors 54, 56 shown in FIG. 1) is above a predefined threshold pressure. In one example, the predefined threshold pressure may be approximately 4-5 g/L loading.

In another embodiment, the controller 60 may be configured to determine whether the thermal regeneration is taking place in the particulate filter 20 by determining whether the present filter temperature (T_(F)) of the particulate filter 20 (as determined by the second temperature sensor 50 shown in FIG. 1) has elapsed a predefined amount of time at a predefined temperature. In one example, the predefined amount of time is 30 minutes and the predefined temperature is 550 Celsius. Any other suitable way of determining when the thermal regeneration is taking place may be employed.

If thermal regeneration is not taking place, the algorithm 200 is exited as indicated by line 210. If thermal regeneration is taking place, the algorithm 200 proceeds to step 204. In step 204 of FIG. 2, the controller 60 controls the temperature difference (T_(S2)−T_(O)) between the present second SCR temperature (T_(S2)) of the second SCR device 26 and a predefined optimal SCR temperature (T_(O)) to be within a predefined threshold during the thermal regeneration of the particulate filter 20. This may be done via sub-steps 204A-C as described below.

In sub-step 204A, the controller 60 determines the present second SCR temperature (T_(S2)) of the second SCR device 26, based on the first temperature sensor 48 operatively connected to the second SCR device 26. In sub-step 204B of FIG. 2, the controller 60 determines whether the temperature difference (T_(S2)−T_(O)) is above or below a predefined threshold.

In sub-step 204C, the controller 60 directs the heat exchange system 22 to transfer heat from the exhaust gas 14 when the temperature difference (T_(S2)−T_(O)) is above the predefined threshold. In one example, the predefined optimal second SCR temperature (T_(O)) is between approximately 200 and 220° Celsius. In another example, the predefined optimal second SCR temperature (T_(O)) is approximately 220° Celsius and the predefined threshold is approximately 10° Celsius. In this case, if the present second SCR temperature (T_(S2)) is above 230° Celsius, the controller 60 directs the heat exchange system 22 to transfer heat from the exhaust gas 14 until the present second SCR temperature (T_(S2)) is within approximately 10° Celsius of the optimal second SCR temperature (T_(O)), or (T_(S2)−T_(O))≦10.

Thus, the exhaust gas treatment system 16 uses the heat exchange system 22 to maintain an optimal temperature of the second SCR device 26 during the thermal regeneration of the particulate filter 20. As shown by line 206, algorithm 200 loops back to step 204 until the temperature difference (T_(S2)−T_(O)) is no longer above the predefined threshold.

In step 208 of FIG. 2, the controller 60 directs the injector 29 to inject the reductant 28 when the temperature difference (T_(S2)−T_(O)) is below the predefined threshold in order to control the NOx emission in the exhaust gas 14. The reductant 28 is configured to travel to the second SCR device 26, where a NOx reduction reaction takes place with the aid of second catalyst 36, thereby reducing the amount of NOx emission in the exhaust gas 14.

The controller 60 may determine the amount of the reductant 28 to be injected by the injector 29 based upon a number of combination factors. The factors may include, but are not limited to, the respective amounts of NOx in the exhaust gas 14 upstream and downstream of the particulate filter 20, the present second SCR temperature (T_(S2)), the amount of first and second catalysts 44, 36 in the first and second SCR devices 24, 26, respectively, and the exhaust flow rate at the exhaust gas inlet tube 18 of the engine 12.

Referring to FIG. 1, the heat exchange system 22 may be a portion of a vehicle exhaust gas heat recovery (EGHR) system or it may be a separate unit installed within the vehicle 10. The heat exchange system 22 may include an inlet portion 62 configured to receive the exhaust gas 14 from the particulate filter 20. An outlet portion 64 of the heat exchange system 22 is configured to transmit the exhaust gas 14 to the second SCR device 26. An interior cavity 66 connects the inlet and outlet portions 62, 64 and defines a central passageway 68 and a bypass passageway 70.

Referring to FIG. 1, the heat exchange system 22 includes a heat exchange device 72 positioned within the bypass passageway 70. The heat exchange device 72 functions as a heat sink within the heat exchange system 22 and is configured to transfer heat from the exhaust gas 14 travelling through the bypass passageway 70. A bypass valve 74 controls flow of the exhaust gas 14 through the heat exchange device 72. The bypass valve 74 may be moved in response to a control signal from a controller 60. The bypass valve 74 may be controlled by a solenoid, a mechanical thermostat, a wax motor, vacuum actuator, or other suitable controls.

Referring to FIG. 1, the bypass valve 74 is selectively movable between a plurality of positions, such as 76A, B and C, to selectively permit the exhaust gas 14 entering the second SCR device 26 to include a first portion 78 from the central passageway 68 and a second portion 80 from the bypass passageway 70. The first and second portions 78, 80 may be anywhere between 0 and 100% of the total amount of the exhaust gas 14. The controller 60 may direct the bypass valve 74 to the first position 76A when the temperature difference (T_(S2)−T_(O)) is below the predefined threshold. In the first position 76A, the first portion 78 may be approximately 100% and the second portion 80 may be approximately 0%, i.e., only exhaust gas 14 from the central passageway 68 is permitted to enter the second SCR device 26.

The controller 60 may direct the bypass valve 74 to the second position 76B when the temperature difference (T_(S2)−T_(O)) is above the predefined threshold. In the second position 76B, the first portion 78 may be approximately 60% and the second portion 80 may be approximately 40%. The controller 60 may also direct the bypass valve 74 to a third position 76C, in which the first portion 78 is approximately 0% and the second portion 80 is approximately 100%. As shown in FIG. 1, the bypass valve 74 may be positioned at the outlet portion 64. The bypass valve 74 may also be positioned at the inlet portion 62. The range of motion of the bypass valve 74 may be varied based on the particular application at hand.

Referring to FIG. 1, the heat exchange system 22 is configured to transfer heat from the exhaust gas 14 to a coolant circuit 82, thereby warming a coolant 84 within the coolant circuit 82. Coolant 84 may flow into and out of the heat exchange system 22 through a coolant inlet port 86 and a coolant output port 88, respectively. The coolant circuit 82 is configured to connect the engine 12 and the heat exchange system 22. The coolant circuit 82 is supplied with pressurized coolant 84 by a primary pump 90 incorporated with the engine 12. The primary pump 90 may be a mechanical pump driven by rotation of the engine crankshaft (not shown). An auxiliary pump 92 may be used to add pressure and increase flow through the coolant circuit 82. The auxiliary pump 92 may be used to supplement the primary pump 90 or may be used as the sole pump in certain situations, for example, when the engine 12 and the primary pump 90 are not operating.

The coolant circuit 82 may transfer heat between various vehicle components, including the engine 12, the exhaust system 16, a heater core 94, and the vehicle transmission (not shown). The heater core 94 allows heat to be transferred from the coolant 84 leaving the engine 12 to the passenger compartment (not shown) of the vehicle 10. The coolant circuit 82 may include a heater core bypass 98 in parallel with the heater core 94, and a heater core bypass valve 96 configured to control flow of coolant 84 through the heater core 94 and the heater core bypass 98. The coolant circuit 82 may include flow restrictors, such as restrictor 99, placed at various locations within the circuit 82. The vehicle 10 may include various other components known to those skilled in the art, including but not limited to, a radiator, transmission heat exchanger and thermostat (not shown).

FIG. 3 is a schematic perspective view of an example heat exchange device 72 that may be employed in the exhaust gas treatment system 16 of FIG. 1. Referring to FIG. 3, the heat exchange device 72 may include a plurality of plates 102 having respective spaces 104 between the plates 102. The respective spaces 104 define a first flow path for the exhaust gas 14. Each of the plates 102 may include one or more respective slots 106A, B, C and D. Referring to FIG. 3, the respective slots 106A-D may be aligned to fit respective tubes 108A-D configured for flow of the coolant 84. The location and number of slots in each plate 102 may be varied based on the particular application at hand. The plates 102 and tubes 108A-D may include corrugations to improve the efficiency of heat transfer. The device shown in FIG. 3 is one example and any suitable type of device known to those skilled in the art may be employed.

The controller 60 of FIG. 1 may include a computing device that employs an operating system or processor for storing and executing computer-executable instructions. Computer-executable instructions may be compiled or interpreted from computer programs created using a variety of programming languages and/or technologies, including, without limitation, and either alone or in combination, Java™, C, C++, Visual Basic, Java Script, Perl, etc. In general, a processor (e.g., a microprocessor) receives instructions, e.g., from a memory, a computer-readable medium, etc., and executes these instructions, thereby performing one or more processes, including one or more of the processes described herein. Such instructions and other data may be stored and transmitted using a variety of computer-readable media. A computer-readable medium (also referred to as a processor-readable medium) includes any non-transitory (e.g., tangible) medium that participates in providing data (e.g., instructions) that may be read by a computer (e.g., by a processor of a computer). Such a medium may take many forms, including, but not limited to, non-volatile media and volatile media. Non-volatile media may include, for example, optical or magnetic disks and other persistent memory. Volatile media may include, for example, dynamic random access memory (DRAM), which may constitute a main memory. Such instructions may be transmitted by one or more transmission media, including coaxial cables, copper wire and fiber optics, including the wires that comprise a system bus coupled to a processor of a computer. Some forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, an EPROM, a FLASH-EEPROM, any other memory chip or cartridge, or any other medium from which a computer can read.

The detailed description and the drawings or figures are supportive and descriptive of the invention, but the scope of the invention is defined solely by the claims. While some of the best modes and other embodiments for carrying out the claimed invention have been described in detail, various alternative designs and embodiments exist for practicing the invention defined in the appended claims. 

1. An exhaust gas treatment system for an engine producing an exhaust gas, the system comprising: an exhaust gas inlet tube configured to receive the exhaust gas; a particulate filter in fluid communication with the exhaust gas inlet tube and configured to undergo thermal regeneration when the exhaust gas in the particulate filter is heated above a regeneration temperature; a heat exchange system in fluid communication with the exhaust gas inlet tube and positioned downstream of the particulate filter; a first selective catalytic reduction (SCR) device in fluid communication with the exhaust gas inlet tube and positioned upstream of the heat exchange system; a second selective catalytic reduction (SCR) device in fluid communication with the exhaust gas inlet tube and positioned downstream of the heat exchange system; a first temperature sensor operatively connected to and configured to determine a present second SCR temperature (T_(S2)) of the second SCR device; a controller operatively connected to the first temperature sensor and configured to determine whether the thermal regeneration is taking place in the particulate filter; and wherein the controller is configured to control a temperature difference (T_(S2)−T_(O)) between the present second SCR temperature (T_(S2)) and a predefined optimal second SCR temperature (T_(O)) to be within a predefined threshold during the thermal regeneration of the particulate filter.
 2. The exhaust gas treatment system of claim 1, further comprising: an injector operatively connected to and configured to selectively inject a reductant into the first SCR device, the reductant being configured to travel to the second SCR device; and wherein the controller is configured to direct the injector to inject the reductant when the temperature difference (T_(S2)−T_(O)) is below the predefined threshold, thereby controlling a NOx emission in the exhaust gas.
 3. The exhaust gas treatment system of claim 1, wherein the predefined optimal second SCR temperature (T_(O)) is between approximately 200 and 220° Celsius.
 4. The exhaust gas treatment system of claim 1, wherein the predefined optimal second SCR temperature (T_(O)) is approximately 220° Celsius and the predefined threshold is approximately 10° Celsius.
 5. The exhaust gas treatment system of claim 1, wherein the controller being configured to control the temperature difference (T_(S2)−T_(O)) to be within a predefined threshold includes: the controller being configured to direct the heat exchange system to transfer heat from the exhaust gas when the temperature difference (T_(S2)−T_(O)) is above the predefined threshold.
 6. The exhaust gas treatment system of claim 1, further comprising: first and second pressure sensors positioned upstream and downstream of the particulate filter, respectively, and configured to determine a differential pressure across the particulate filter; and wherein the controller is configured to determine whether the thermal regeneration is taking place in the particulate filter by determining when the differential pressure across the particulate filter is above a predefined threshold pressure.
 7. The exhaust gas treatment system of claim 1, further comprising: a second temperature sensor operatively connected to and configured to determine a present filter temperature (T_(F)) of the particulate filter; and wherein the controller is configured to determine whether the thermal regeneration is taking place in the particulate filter by determining whether the present filter temperature (T_(F)) of the particulate filter has elapsed a predefined amount of time at a predefined temperature.
 8. The exhaust gas treatment system of claim 7, wherein the predefined amount of time is 30 minutes and the predefined temperature is 550 Celsius.
 9. The exhaust gas treatment system of claim 1, wherein: the first SCR device includes a first catalyst; the particulate filter includes a plurality of channels having respective walls; and the first SCR device and the particulate filter are disposed in a common housing such that the first catalyst is coated on the respective walls of the plurality of channels of the particulate filter.
 10. The exhaust gas treatment system of claim 1, further comprising: first and second NOx sensors positioned upstream and downstream of the particulate filter, respectively; and wherein the first and the second NOx sensors are configured to determine respective amounts of the NOx emission in the exhaust gas upstream and downstream of the particulate filter.
 11. The exhaust gas treatment system of claim 1, wherein the heat exchange system includes: an inlet portion configured to receive the exhaust gas from the particulate filter; an outlet portion configured to transmit the exhaust gas to the second SCR device; an interior cavity connecting the inlet and outlet portions and defining a central passageway and a bypass passageway; a heat exchange device positioned within the bypass passageway and configured to transfer heat from the exhaust gas; and a bypass valve selectively movable between a plurality of positions to selectively permit the exhaust gas entering the second SCR device to include a first portion from the central passageway and a second portion from the bypass passageway.
 12. The exhaust gas treatment system of claim 11, wherein the bypass valve is positioned such that the first portion is approximately 100% and the second portion is approximately 0% when the temperature difference (T_(S2)−T_(O)) is below the predefined threshold.
 13. The exhaust gas treatment system of claim 11, wherein the bypass valve is positioned such that the first portion is approximately 60% and the second portion is approximately 40% when the temperature difference (T_(S2)−T_(O)) is above the predefined threshold.
 14. The exhaust gas treatment system of claim 11, wherein the heat exchange device includes: a plurality of plates having respective spaces between the plurality of plates, the respective spaces defining a first flow path for the exhaust gas; wherein the plurality of plates each define at least one respective slot, the at least one respective slot being aligned to fit at least one tube configured for flow of a coolant.
 15. The exhaust gas treatment system of claim 8, further comprising: a coolant circuit operatively connected to the heat exchange system; and wherein the heat exchange device is configured to selectively transfer heat from the exhaust gas to the coolant circuit.
 16. A vehicle comprising: an engine; an exhaust gas inlet tube in fluid communication with and configured to receive an exhaust gas from the engine; a particulate filter in fluid communication with the exhaust gas inlet tube and configured to undergo thermal regeneration when the exhaust gas in the particulate filter is heated above a regeneration temperature; a heat exchange system in fluid communication with the exhaust gas inlet tube and positioned downstream of the particulate filter; a first selective catalytic reduction (SCR) device in fluid communication with the exhaust gas inlet tube and positioned upstream of the heat exchange system; a second selective catalytic reduction (SCR) device in fluid communication with the exhaust gas inlet tube and positioned downstream of the heat exchange system; an injector operatively connected to and configured to selectively inject a reductant into the first SCR device, the reductant being configured to travel to the second SCR device; a first temperature sensor operatively connected to the second SCR device and configured to determine a present temperature of the second SCR device; a controller operatively connected to the first temperature sensor and configured to determine whether the thermal regeneration is taking place in the particulate filter; wherein the controller is configured to control a temperature difference (T_(S2)−T_(O)) between the present second SCR temperature (T_(S2)) and a predefined optimal second SCR temperature (T_(O)) to be within a predefined threshold during the thermal regeneration of the particulate filter; and wherein the controller is configured to direct the injector to inject the reductant when the temperature difference is below the predefined threshold, thereby controlling a NOx emission in the exhaust gas.
 17. A method for controlling operation of an exhaust gas treatment system in an engine producing exhaust gas, the method comprising: operatively connecting an exhaust gas inlet tube to the engine for receiving the exhaust gas; operatively connecting a particulate filter and heat exchange system for fluid communication with the exhaust gas inlet tube, the particulate filter being configured to undergo thermal regeneration when the exhaust gas in the particulate filter is heated above a regeneration temperature; operatively connecting first and second selective catalytic reduction (SCR) devices for fluid communication with the exhaust gas inlet tube, the first and second SCR devices being positioned upstream and downstream of the heat exchange system, respectively; operatively connecting an injector to the first SCR device for selective injection of a reductant into the first SCR device; detecting when the thermal regeneration is taking place in the particulate filter; controlling the temperature difference between the present SCR temperature and a predefined optimal SCR temperature to be within a predefined threshold during the thermal regeneration of the particulate filter; and directing the injector to inject the reductant when the temperature difference is below the predefined threshold, thereby controlling a NOx emission in the exhaust gas.
 18. The method of claim 17, wherein controlling the temperature difference to be within a predefined threshold during the thermal regeneration of the particulate filter includes: determining a present second SCR temperature of the second SCR device based on a first temperature sensor operatively connected to the second SCR device; determining whether the temperature difference between the present SCR temperature and a predefined optimal SCR temperature is above or below the predefined threshold when the thermal regeneration is taking place in the particulate filter; and directing the heat exchange system to transfer heat from the exhaust gas when the temperature difference is above the predefined threshold.
 19. The method of claim 16, further comprising: positioning first and second pressure sensors upstream and downstream of the particulate filter, respectively, for determining a differential pressure across the particulate filter; and wherein said detecting when the thermal regeneration is taking place in the particulate filter includes determining when the differential pressure is above a predefined threshold pressure. 